The present application relates to assessing the integrity of a downhole cement.
Subterranean formation operations (e.g., stimulation operations, sand control operations, completion operations, etc.) often involve drilling a wellbore in a subterranean formation with a drilling fluid (and thereafter placing a cement column between the formation and a casing (or liner string) in the wellbore. The cement column is formed by pumping a cement slurry through the bottom of the casing and out through an annulus between the outer casing wall and the formation face of the wellbore, or by directly pumping a cement slurry into the annulus. The cement slurry then cures in the annular space, thereby forming a column of hardened cement that, inter alia, supports and positions the casing in the wellbore and bonds the exterior surface of the casing to the subterranean formation. This process is referred to as “primary cementing.”
Among other things, the cement column may keep fresh water reservoirs from becoming contaminated with produced fluids from within the wellbore. As used herein, the term “fluid” refers to liquid phase fluids and gas phase fluids. The cement column may also prevent unstable formations from caving in, thereby reducing the chance of a casing collapse and/or stuck drill pipe. Finally, the cement column forms a solid barrier to prevent fluid loss or contamination of production zones. The degree of success of a subterranean formation operation involving placement of a cement column, therefore, depends, at least in part, upon the successful cementing of the wellbore casing and the cement's ability to maintain zonal isolation of the wellbore.
Failure of zonal isolation, among other things, may result in environmental contamination, which may cause harm to both flora and fauna, including humans. Such failure may further prevent production or reduce the production capability of a wellbore, which may result in abandonment. These issues may become exacerbated over time, where an understanding of the state of the cement column at an earlier point in time (e.g., the physical attributes of defects in the cement) may allow remedial actions to be performed and abandonment avoided.